The present invention relates to a carbon fiber having a structure that the outer peripheral region of the fiber cross section is composed of an optically isotropic component and its central region is composed of an optically anisotropic component or an optically anisotropic component partially containing optically isotropic component, which the fiber surface shows no cracks at all, and a process for producing the same.
Carbon fibers are widely in use in the field of special structural parts in space industry and aircraft industry and in the field of leisure articles and sport articles. 2. Brief Description of the Prior Art
Generally saying, carbon fibers are roughly classified into general-purpose carbon fibers and high-performance carbon fibers according to their mechanical properties.
The high-performance carbon fibers are further classified according to their starting material into those using synthetic fiber such as polyacrylonitrile and the like as starting material and those using petroleum pitch and coal tar pitch as starting material.
When a synthetic fiber such as polyacrylonitrile is used as starting material, the cost of product is unavoidably high due to the high price of starting synthetic fiber and the low carbonization yield from the starting fiber, and this high cost has made the most important cause obstructing the growth of this type of carbon fibers as general industrial materials.
In such a state of things, the process for producing high-performance carbon fiber at a low cost by using the optically anisotropic mesophase pitch as starting material is being studied. (If a substance having polynuclear polycyclic molecule such as pitch is made to grow by heat treatment, the whole or a part of the pitch becomes exhibiting a state of liquid crystal. Such a liquid state is called "carbonaceous mesophase" or simply "mesophase". A pitch containing such mesophase is called "mesophase pitch". It is anisotropic optically.)
Regarding the content of mesophase in mesophase pitch, it is mentioned to be 40 -90% or 70% by weight or more in Japanese Patent Publication No. 37,611/80 and Japanese Patent Publication No. 51,526/83 (corresponding to U.S. Pat. No. 4,301,135). We may consider that the term "mesophase pitch" inclusively means the pitches containing mesophase in these amounts.
According to the prior techniques, a carbon fiber prepared by spinning such a mesophase pitch at a low temperature (300 to 350.degree. C.) has had a fault that cracks formed on the surface of fiber greatly deteriorate the performances of carbon fiber.
The mesophase, i.e. the, anisotropic component of pitch, is oriented in the direction of fiber axis at the time of melt spinning. It is known that the texture of fiber section perpendicular to fiber axis is classified into three types according to direction of the orientation.
Said three types are "radial structure" perpendicular to fiber axis, "onion structure" constituted of concentric orientation and "random structure" constituted of irregular orientation. Hitherto, these structures have been considered dependent on spinning temperature. In other words, it is considered that the structure changes from radial to random and further to onion as the spinning temperature rises. In FIG. 1, (1-1) denotes radial structure having cracks on fiber surface; (1-1) expresses random structure, and (1-3) does onion structure.
Since a fiber of radial structure readily cracks at the fiber surface, such a fiber is disadvantageous as a general industrial material. Further, since such a crack brings about a decrease in strength, radial structure is considered undesirable. Thus, a number of proposals have hitherto been made about the technique for producing carbon fibers having random or onion structure not forming cracks on fiber surface, and more particularly the technique for spinning such carbon fibers.
These proposals can be roughly classified into the following two methods:
(1) A method for obtaining carbon fiber having random or onion structure by employing a high spinning temperature. PA1 (2) A method for obtaining carbon fiber having random or onion structure by controlling the flow of molten pitch passing the spinning nozzle.
As one example of Method (1), Japanese Patent Kokai (Laid-Open) No. 76,925/84 (corresponding to G.B. 2,131,781) can be referred to. According to this process, the phase states of anisotropic component and isotropic component at the spinning temperature are regarded as the factor determining the structure of fiber, and carbon fiber having random or onion structure is produced by carrying out the spinning at a temperature at which matrix becomes isotropic (the term "matrix" means the phase playing the role of parent phase in a two phase mixture consisting of isotropic component and mesophase.). Another process belonging to Method (1) is disclosed in Japanese Patent Kokai (Laid-Open) No. 53,717/84 (corresponding to U.S. Pat. No. 4,590,055 combining with Japanese Patent Kokai Nos. 36,724/84, 36,725/84, 36,726/84 and 53,717/84) according to which a carbon fiber having a structure free from cracks formation on its fiber surface is obtained by heating the starting pitch to a temperature higher than its viscosity-change temperature and thereafter spinning it.
However, all these processes cannot be said to be desirable from the viewpoint of stability of spinning process because of high spinning temperature which causes formation of bubbles in molten pitch. Thus, the bubbles cause breakage of fiber at the time of spinning, and the bubbles are sometimes taken into fiber. Further, these processes cannot be applied to the spinning of multi-filament, filament, because melt viscosity is low in these processes.
On the other hand, as example of Method (2), the processes of Japanese Patent Kokai (Laid-Open) No. 168,124/84, Japanese Patent Kokai (Laid-Open) No. 168,127/84 (corresponding to W.O. 8,403,722), etc. can be referred to. According to these processes, the flow of anisotropic pitch is controlled so as to form a carbon fiber of random structure or onion structure by varying the shape of nozzle. However, these processes can exhibit their effect only when the molten pitch has a good flowability, and therefore a high spinning temperature is similarly necessary substantially, due to which bubble formation of molten pitch and contamination of bubbles into fiber are unavoidable. Further, these processes have another disadvantage that the nozzle for these processes is difficult to produce.
As above, all the prior processes for producing carbon fiber having a structure free from the formation of cracks on its fiber surface have been based on a mechanical control of the flow of molten pitch passing through nozzle by some means such as lowering the melt viscosity of pitch, altering the shape of nozzle, or the like. As the result, these processes can exhibit their effect only at a high spinning temperature. In other words, in all these processes, the spinning is carried out in a temperature region at which molten pitch is thermally instable. Accordingly, the resulting fiber is broken due to bubbles at the time of spinning or contains bubbles, and the processes are unsatisfactory and unable to produce a high performance carbon fiber stably on an industrial scale.